The present invention utilizes a solid electrolyte sensor for detection of certain gases. The Nernst equation describes the behavior of sensing devices using solid electrolytes. When two media with different partial pressures, P.sub.1 and P.sub.2, of a particular substance present in both media are separated by a solid electrolyte (ionic conductor) and conducting electrodes are attached to both sides of the ionic conductor, an EMF is generated which is related to the partial pressure as follows: ##EQU1## where R is the gas constant, T is absolute temperature, F is the Faraday constant, E.sub.o is the standard oxidation-reduction potential difference, EMF is electromotive force, and n is the number of electrons per molecule of product from the overall cell reaction.
If the system described by the above equation behaves nonideally, the partial pressures must be replaced by fugacities. Another factor which may need to be considered in regard to a particular system is the rate of dissociation to form the ions which pass through the solid electrolyte. This may be a limiting factor to the transfer of ions through the electrolyte. The monofunctional monomer which is employed to form the guest polymer differs from any of the monomeric repeat units which comprise the organic polymer in the host polymer blend so that an interpenetrating polymer network may be formed. The rate of dissociation can be calculated by means of the equilibrium constant for the dissociation reaction.
The magnitude of EMF produced is generally in accordance with the parameters discussed herein: the Nernst equation and, where applicable, the dissociation equilibrium constant. However, required practice in measuring concentration is to periodically calibrate the measuring apparatus by use of samples whose composition is known. Thus, exact adherence to theoretical relationships is not required of commercially used methods and apparatus. The primary commercial requirement is repeatability.
In a majority of cases, the admixture of an organic compound, especially in a polymeric state, with an inorganic compound, results in a phase separation, due to the fact that the two systems are immiscible in nature. However, in a macroscopically homogeneous polymer blend, which we term a host polymer blend, may be prepared by admixing organic and inorganic components as discussed herein; the resulting substance (the host polymer blend) is not merely a physical mixture but exhibits a degree of interaction, that is, some amount of chemical interaction exists. The host polymer blend is then admixed with a monomer and a difunctional cross-linking agent in a compatible solvent. The mixture is cast on a smooth surface and the solvent removed to form a membrane. The monomer is polymerized to form a polymer termed the guest polymer, thereby forming an interpenetrating polymer network membrane.
Interpenetrating polymer networks are known to those skilled in the art. There are three general classes of interpenetrating polymer networks: sequential, simultaneous, and latex. The classes differ in method of preparation. In order to prepare an interpenetrating polymer network of the sequential class, a first polymer, termed a host polymer, is prepared in the absence of a second monomer. The host polymer is then mixed with a second monomer and a cross-linking agent in a compatible solvent. After removal of the solvent, the second monomer and cross-linking agent are polymerized to yield the guest polymer of the interpenetrating polymer network and the guest polymer is cross-linked to itself. An interpenetrating polymer network is more than a blend of two polymers. It is a new polymer system having properties which are a combination of those possessed by the host polymer and the guest polymer. However, there is no chemical bonding between the host and guest polymers. In the new polymer system, the host and guest polymer chains are permanently entangled, or intertwined, with one another. It is impossible to separate the two polymers by chemical methods (such as leaching or dissolving one polymer away from the other). It can be seen that the present invention may be termed an interpenetrating polymer network.
Substances which are permeable by gases in a selective manner are known and utilized in a variety of applications. A membrane formed in accordance with the present disclosure is substantially impermeable to ions and gases, including hydrogen gas, but does not allow hydrogen ions to pass through it. For background information relating to the principles of the present invention, reference may be made to the book Solid Electrolytes and Their Applications, edited by Subbarao, Plenum Press, 1980.
Low mechanical strength has been a common problem when attempting to apply permselective membranes. The present invention provides a membrane whose mechanical strength is increased by compositing it with other materials, but whose desirable properties are not lost as a result of doing so.
Also used in the present invention is a solid substance which is a substitute for a reference gas, which reference gas is one of the two media mentioned above in the discussion of the Nernst equation. It is highly desirable to use a solid reference substance, which requires only periodic replacement, instead of maintaining a continuous reference gas flow, or in appropriate situations, maintaining a sealed chamber of reference gas. The reference substance is in intimate contact with the catalytic agent on the reference side of the membrane. One substance may serve as both reference substance and catalytic agent.